The invention addresses some of the fundamental issues relevant to the performance objectives for long reach digital subscriber line (LDSL) systems (sometimes referred to as last mile DSL). The objective requirements typically are to provide minimum payload bit rates of 192 kb/s downstream and 96 kb/s upstream on loops that span the equivalent working length of, for example, 18 kilofeet on a 26 AWG (American Wire Gauge) communication line. The crosstalk noise environments for such systems are varied, and may include near-end crosstalk (NEXT) and far-end crosstalk (FEXT) disturbance from ISDN, HDSL, SHDSL, T1, and self-disturbers at both the central office (CO) and customer premise equipment (CPE) ends. NEXT from HDSL and SHDSL tend to limit the performance in the upstream channel while NEXT from T1 systems tend to severely limit the downstream channel performance.
Conventional approaches to LDSL rely on different upstream and downstream power spectral density (PSD) masks that exhibit complementary features. At the modem start up, based on a particular telecommunication standard or protocol, an upstream-downstream pair of PSD masks is selected and implemented. For example, the International Telecommunication Union (ITU)'s RE-ADSL2 standard (also known as ADSL2 Annex L standard), contains specifications for reach-extended operation of ADSL2 systems operating over plain old telephone system (POTS) connections. ADSL2 Annex L contains one mandatory downstream PSD mask, one optional downstream PSD mask, and two mandatory upstream PSD masks. However, in RE-ADSL2 standard and other conventional implementations, the upstream and downstream PSD masks are preformed or preconfigured and thus may not optimally conform to constant power constraints and/or be spectrally compatible with the signal environment in which the LDSL system is operating. For example, since RE-ADSL2 is designed to increase the reach of ADSL while in compliance with the North American Copper Network spectral compatibility regulations, RE-ADSL2 may not meet the long reach demands in other countries where both spectral compatibility rules and field loop/impairments may significantly differ from those in North America. Even in North America, RE-ADSL2 does not always guarantee an optimal set of PSD masks due to varying signal environments in different long reach implementations.
In view of the foregoing, a need exits for a technique for real-time formation of optimal PSD masks which overcomes the above-described inadequacies and shortcomings.